In a typical permanent magnet AC motor, such as a brushless DC motor or a permanent magnet AC servo motor system, for example, the motor winding is on the stator and is in a three-phase configuration. Energization of the windings is controlled through a six-transistor bridge circuit, where the transistors are operated in a switching mode according to the motor position, or the position of the rotor in a rotary motor, and motor force commands, such as torque commands. Three of the transistors are connected to the positive supply bus and the remaining three transistors are connected to the negative supply bus. Sinusoidal or trapezoidal excitation of the winding may be achieved by pulse width modulation (PWM) control of the switching transistors. A back-biased diode, or flyback diode, is commonly connected across each of the emitter-collector circuits of the switching transistors to bypass transients from the switching control of the inductive motor load. The motor windings can also be energized in a controller manner by other amplifier topologies such as a linear transistor amplifier topology.
In motor drive systems of the type discussed above, when the control system fails, it is often necessary to actively bring the motor to zero speed assuming the minimum possible amount of hardware/control is working. In order to do so, many motor control systems include dynamic braking capability. In a dynamic braking mode, the motor usually operates as a generator and either dissipates energy into the motor windings, or dissipates energy into a resistive load. Such dynamic braking may be achieved in both brush-type motors as well as brushless motors, or AC servo motors.
One simple way to brake a motor is to use a relay to disconnect the motor form the power source (electronic drive or AC line) and then put a three phase short across the motor windings. In this manner, the kinetic energy of the motor is dissipated in the motor coil resistances.
Another prior art technique is disclosed in U.S. Pat. No. 6,118,241 to Kazlaukas entitled “Dynamic Braking System for Electrical Motors.” Therein, the electronic drive power transistors short the motor terminals line to line. Dynamic braking is achieved by simultaneously rendering conductive the three transistors connected to a positive bus, or the three transistors connected to a negative bus. This technique is referred to as a “three phase short” technique. When three such transistors are simultaneously rendered conductive, current flows from one or more motor windings to one of the supply busses through one or two of the conductive transistors and returns to other motor winding or windings through one or more of a plurality of back-biased diodes. This arrangement provides dynamic braking regardless of rotor position. The braking operation may be achieved using the three transistors connected to the positive bus or by using the three transistors connected to the negative bus. If sufficient control is provided, the dynamic breaking system may alternate between transistors connected to the positive bus and those connected to the negative bus, thus sharing the load between all transistors. When the three transistors connected to a bus are simultaneously rendered conductive, they essentially short circuit the winding and dynamic braking is provided. When all 6 transistors are simultaneously rendered non-conductive motor current flows through one or more of the plurality of normally back biased diodes and the magnitude of the dynamic braking current is reduced. Pulse width modulation (PWM) control of the conductive intervals may be used to control the degree of braking through controlling the magnitude of the braking current. This transistor shorting modulation technique adds the benefit of allowing the peak current flowing in the motor as a result of the three phase short to be limited in a controlled manner. Limiting the maximum current limits possible damage to the power transistors and also prevents excessive current in the motor which might demagnitize it.
However, one major disadvantage of these aforementioned techniques is that the current that flows when the short occurs does not all go towards stopping the motor. In a three phase permanent magnet motor, at higher speeds, the short circuit current is almost entirely set by the back electromotive force (“back EMF”) applied over the motor winding inductance. The back EMF is the voltage that occurs in electric motors where there is relative motion between the windings of the motor and the external magnetic field from permanent magnets or electro magnets. The back EMF is in quadrature (ninety degrees out of phase) to the current that flows in the motor winding inductance. Since the short circuit current is almost entirely set by the back EMF divided by winding inductive impedance at higher speeds, the short circuit current is almost entirely in quadrature to the back EMF, meaning almost all the current does not create stopping torque.
Stopping the motor as fast as possible in case of a control fault is advantageous, particularly at high speeds. If the motor does not stop fast enough there may be damage to the machine incorporating the motor or even human injury. If the current control circuitry or current limiting circuitry of the motor drive does not act to stop the motor fast enough, the motor drive may fault due to an over current condition causing the braking to stop. Additionally, if the motor is operating at high speeds and the motor is shorted according to the prior art techniques, the resulting current may be high enough that damage may be caused to the windings of the motor or the permanent magnet, or even to the motor drive.
Thus, there remains a need for a system for efficiently stopping a motor in a dynamic braking mode that avoids the problems of the previous systems, including those described above, and uses the minimum amount of control circuitry to insure high reliability. In particular, it would be useful to maximize the stopping torque per ampere of the dynamic braking current to more efficiently brake the motor.